1. Field of the Invention
The invention relates to a method of providing a corrosion resistant coating film on an anodically oxidized surface film on an article composed of aluminum or an aluminum-based alloy.
2. Prior Art
In the prior art, several methods are known for sealing micropores or pinholes and the like in an anodically oxidized surface film of an article composed of aluminum or an aluminum-based alloy, including sealing with pressurized steam, sealing with boiling water as well as sealing with chemicals, such as certain salt compounds, for example nickel acetate, cobalt acetate and the like. When the anodically oxidized surface film of an article composed of aluminum or an aluminum-based alloy is subjected to sealing by the above prior art methods, such surface film is then overcoated with a layer of a coating composition and the underlying anodically oxidized surface film tends to crack or the like during application of such layer. Such cracks or surface discontinuities are caused during the drying process utilized with the coating compositions wherein drying is usually undertaken at a temperature of 140.degree. C. or higher and results in an inferior adhesion of the coating film to the underlying surface, yields an inferior appearance and inferior mechanical properties of the coating film and provides a relatively poor corrosion resistance to the thus-coated article. Therefore, it is a generally accepted practice in the art to seal aluminum articles by coating such articles with a low temperature-drying coating composition curable at temperatures of 140.degree. C. or lower, in spite of the disadvantageous properties of coating films obtained from such low temperature-drying coating compositions in comparison with films obtained from high temperature-drying coating compositions. In attempting to balance these problems, selection of a coating composition useful on aluminum articles is subject to narrow limitations and the properties of the coating films, i.e. adhesion to the underlying surface and corrosion resistance are never completely satisfactory.
As is well known, on the other hand, coating with a high temperature-drying coating composition is usually preceded by a sealing of micropores and the like in the anodically oxidized surface film with a synthetic resin. Such sealing treatment may occur by means of electrodeposition or dipping, prior to overcoating with a select high temperature-drying coating composition. With this type of treatment and with the simultaneous hydration sealing, utilizing the moisture within the coating composition and a high temperature curing or drying, crack formations and the like are avoided. However, a small amount of sealing liquid, for example, sulfuric acid, often remains adsorbed in the micropores within the unsealed or partially sealed anodically oxidized surface film. An aluminum article having such a sealing film on an anodically oxidized surface film is defective due to the poor corrosion resistance as well as the low wear resistance and poor durability and adhesion of the coating film.
In addition, electrolytic coloring of an anodically oxidized surface film, for example, in accordance with the method suggested by Asada (Japanese Patent Publication No. 38-1715), wherein a metal oxide at a lower oxidation state is deposited electrolytically on the anodically oxidized surface film, causes yet further problems, i.e. a degradation of any coating film applied on top of such colored aluminum occurs. It appears that the degradation of the coating film may be caused by a migration of the coloring substances out of the micropores or by a migration of the metal into the electro-deposited coating films.
Processes of improving the corrosion resistance of articles comprised of aluminum or an aluminum-based alloy by coating surfaces thereof with compositions, which are either of the high temperature-drying type or the low temperature-drying type, are defective in many ways, particularly by failing to provide coating films having desired properties, such as good adhesion to the underlying surface, good wear resistance and the like, good resistance against alkali solution, hydrochloric acid, saline solution, sulfurous acid solutions etc., and good weathering resistance on outdoor exposure. The prior art procedures apparently fail to completely seal micropores and the like in the anodically oxidized surface film on articles comprised of aluminum or an aluminum-based alloy and thus yield coated articles with inferior properties.